Such compound sonotrodes have been described in particular in U.S. Pat. No. 28 642 issued to Dukane Corporation, and in French Patent No. 73 12 673 assigned to the assignee of the present application. In FIG. 1 of the accompanying drawings is shown an embodiment of a compound sonotrode of the prior art.
Such compound sonotrode is composed substantially of a carrier block 10, tuned to the half-wavelength of the nominal frequency of an ultrasonic transmitter or transducer 12 (the frequencies generally selected being of the order of 20 to 40 Hz), a plurality of sonotrodes or concentrators 14, cut off to the half-wavelength at the same frequency as block 10, and pins 16 for the fixation of the concentrators 14 to the carrier block 10.
The assembly thus formed provides a system in two half-wavelengths. In FIG. 1, the amplitude loops have been denoted by reference V, and the amplitude nodes, or stress loops, have been denoted by reference N. Such a known system of compound sonotrodes allows notably concentration of energy on the active area of the sonotrodes 14, determined according to welding areas to be formed on an article to be assembled.
Such a known system has the disadvantage of being only applicable in the case where it is possible to screw the sonotrodes or concentrators 14, onto the carrier block 10 via pins 16, thereby making impossible a compound sonotrode comprising two adjacent rectangular blocks mounted on the same block or adaptor 10, since one cannot screw the two rectangular blocks in order to fix them on an adaptor 10.
In order to remedy the disadvantages of this known solution, it has been envisaged to provide compound sonotrodes comprising concentrators of rectangular shape, mounted side by side. This solution, which also is within the prior art, is been shown in FIGS. 2 and 3 of the accompanying drawings.
One sees that the compound sonotrode of this type if comprised of a supporting block 18 cut off to the half-wavelength, at least two concentrators 20, 20' having the shape of rectangular blocks and arranged side by side in positions preventing any rotation about fixation means, and fixation pins 22, 22'. As is shown in FIG. 3, each fixation pin 22 (or 22') comprises on one side a right-hand thread 24, on the other side a left-hand thread 26, and an intermediate portion 28 of hexagonal shape for rotation to thread the concentrator blocks 20, 20' toward and onto the carrier block 18.
With this solution, it is actually possible to set in vibration several concentrator blocks, such as 20, 20', on the same adaptor block 18, but under the prerequisite that the fixation pins 22, 22' do not disturb the vibratory mode of operation of the thus formed compound sonotrode. But experience shows that the weight of the fixation pins such as 22, 22' introduces disturbances in the vibratory mode of operation of such compound sonotrodes, and that the pins inevitably break after a certain period of operation. This disadvantage, which is foreseeable, is due to the fact that each fixation pin forms a significant and not tuned mass, subjected to intensive vibrations since it is located in the area of maximum amplitude, and that such mass under no circumstances can tune in to the vibratory system formed by the compound sonotrode.
Therefore, such a known technique does not allow obtaining compound sonotrodes adapted for working according to the half-wavelength theoretical mode of operation, since the presence of the significant mass formed by the fixation pin, or amplitude loop V of the resonant system, seriously disturbs the resonance mode of operation and introduces side stresses at the level of the fixation of the pins, where precisely there should be the minimum of stresses.